MAMMALS. 361 



All the parts of animal structure are in beautiful harmony with 

 each other and with the habits and instincts of the species. The 

 short and powerful jaw in the cats (Fdidce), the lacerating teeth, 

 the muscular fore limbs, their freedom of motion, the sharp curved 

 talons, the flexibility of the spine, and the straight and simple 

 digestive canal, equally indicate activity and testify to the posses- 

 sion of sanguinary and carnivorous propensities. In the camels, 

 the prominent lips, the structure of the teeth, the broad spongy 

 soles of the feet, the callous pads on the limbs, the complex diges- 

 tive apparatus, and the water-cells, all point out a creature fitted 

 for feeding on coarse and thorny herbage and for traversing sandy 

 deserts. Neither of these animals could exchange any portion of 

 its structure with the other without serious derangement of the 

 whole. This correspondence of part with part, and the adaptation 

 of every organ to the mode of life prescribed, is so exactly main- 

 tained, that a skilful comparative anatomist can, from a single 

 tooth or bone, build up in imagination the whole structure of an 

 animal which he never saw, indicate its form, and pronounce with 

 considerable confidence upon its food, its habits, and its manner 

 of life. 



The classification of the Mammals is based upon the structure 

 of their teeth and feet. These are the organs that most affect the 

 conditions of existence under which each is found. 



There is, however, one group, almost limited in geographical 

 extent to Australia and its islands, so peculiarly organized, that 

 they may be considered as forming a connecting-link between the 

 true Mammals and the Oviparous Vertebrata, and to these re- 

 markable quadrupeds we must next beg the reader's attention. 



SUB-CLASS O VO-VIVIPARA.* 

 ORDER I. MoxoTREMATA.f 



The broad characters whereby a bird is distinguishable from a 

 mammiferous animal, as we have endeavoured to show, are plain 

 and simple enough. The bird lays eggs and incubates them ; the 

 Mammal produces its young alive, and suckles them ; yet, strange 

 to say, there are certain creatures so exactly intermediate in their 

 organization between these two great classes, 'that even the ana- 



* Ovum, an egg ; viviparus, giving birth to living offspring: so called because it is pro- 

 blematical whether they produce eggs or living young. 



f /i6foj, monos, single ; Tprj/j.a, trema, an orifice i.e., having a single excretory and 



generative outlet. 



